1. Technical Field
The subject matter described here generally relates to fluid reaction surfaces with specific blade structures and, more particularly, to wind turbines blades tip shapes.
2. Related Art
A wind turbine is a machine for converting the kinetic energy in wind into mechanical energy. If the mechanical energy is used directly by the machinery, such as to pump water or to grind wheat, then the wind turbine may be referred to as a windmill. Similarly, if the mechanical energy is converted to electricity, then the machine may also be referred to as a wind generator or wind power plant.
Wind turbines are typically categorized according to the vertical or horizontal axis about which the blades rotate. One so-called horizontal-axis wind generator is schematically illustrated in FIG. 1 and available from General Electric Company. This particular configuration for a wind turbine 2 includes a tower 4 supporting a nacelle 6 enclosing a drive train 8. The blades 10 are arranged on a hub to form a “rotor” at one end of the drive train 8 outside of the nacelle 6. The rotating blades 10 drive a gearbox 12 connected to an electrical generator 14 at the other end of the drive train 8 arranged inside the nacelle 6 along with a control system 16 that receives input from an anemometer 18.
The blades 10 generate lift and capture momentum from moving air that is then imparted to a rotor as the blades spin in the “rotor plane.” Each blade is typically secured at its “root” end, and then “spans” radially “outboard” to a free, “tip” end. The distance from the tip to the root, at the opposite end of the blade, is called the “span.” The front, or “leading edge,” of the blade connects the forward-most points of the blade that first contact the air. The rear, or “trailing edge,” of the blade is where airflow that has been separated by the leading edge rejoins after passing over the suction and pressure surfaces of the blade.
A “chord line” connects the leading and trailing edges of a cross section of the blade 10 that is oriented normal to the radial direction. The length of the chord line is simply referred to as the “chord.” Since many blades 10 change their chord over the span, the chord length is referred to as the “root chord,” near the root, and the “tip chord,” near the tip of the blade. The chord lines are arranged in the “chord planes” that extend through the corresponding pressure and suction surfaces of the blade. The center of the chord plane, or “chord plane center line,” is formed by a line on the chord plane which is halfway between the leading and trailing edge of the blade 10. Multiple “shear web planes” are arranged perpendicular to the chord plane.
The resulting shape of the blade 10, when viewed perpendicular to the direction of flow, is called the “planform.” For example, FIG. 2 is schematic planform view of the tip portion of the blade 10 shown in FIG. 1 where the tip chord 20 forms a “leading edge tip chord angle” α with the leading edge 22, and a “trailing edge tip chord angle” β with the trailing edge 24.
The noise and power performance of the wind turbine blades 10 depends, in part, upon vortex development at the tip of the blade. Various techniques have been proposed to control this vortex development. For example, commonly-owned co-pending U.S. application Ser. No. 11/827,532 filed on Jul. 12, 2007 discloses a wind turbine blade having a vortex breaking system for reducing noise while commonly-owned co-pending U.S. Application Serial No. 12129997 filed on May 30, 2008 discloses wind turbine blade planforms with twisted and tapered tips. While vortex development can generally be reduced by minimizing the aerodynamic load at the tip of the blade, so-called “tip unloading” typically causes a significant reduction in power that is produced by the blade.